Pelagic barite precipitation at micromolar ambient sulfate

Abstract

Geochemical analyses of sedimentary barites (barium sulfates) in the geological record have yielded fundamental insights into the chemistry of the Archean environment and evolutionary origin of microbial metabolisms. However, the question of how barites were able to precipitate from a contemporary ocean that contained only trace amounts of sulfate remains controversial. Here we report dissolved and particulate multi-element and barium-isotopic data from Lake Superior that evidence pelagic barite precipitation at micromolar ambient sulfate. These pelagic barites likely precipitate within particle-associated microenvironments supplied with additional barium and sulfate ions derived from heterotrophic remineralization of organic matter. If active during the Archean, pelagic precipitation and subsequent sedimentation may account for the genesis of enigmatic barite deposits. Indeed, barium-isotopic analyses of barites from the Paleoarchean Dresser Formation are consistent with a pelagic mechanism of precipitation, which altogether offers a new paradigm for interpreting the temporal occurrence of barites in the geological record.

Fig. 1

Map showing cruise track, underway temperature, and sampling locations in Lake Superior. Dissolved and particulate samples were collected at FWM (46°59′54.7″ N, 91°14′46.5″ W; 144 m water depth; 26 August 2015) and WM (47°19′53.8″ N, 89°49′17.0″ W; 180 m water depth; 27 August 2015) aboard the R/V Blue Heron (cruise BH15-11, Duluth MN–Duluth MN). Map drafted using GeoMapApp (http://www.geomapapp.org); Lake Superior bathymetry from National Centers for Environmental Information (https://www.ngdc.noaa.gov)

Fig. 2

Multi-element geochemistry from Lake Superior. Particulate data were obtained by filtering between 0.5–3.0 L of Lake Superior water through a 0.45 μm polyethersulfone membrane filter immediately after collection. a–d from St. FWM; e–h from St. WM; dashed and solid lines show particulate and dissolved samples, respectively. Vertical error bars on the shallowest samples in d, h denote depth ranges over which particulate samples were pooled to obtain sufficient Ba for isotopic analysis. Horizontal error bars for any given property measurement reflect the propagated 2 × SD uncertainty. Thin dotted lines in a, e illustrate power-law fits to p[P] profiles, excluding the benthic nepheloid layer sample from FWM (see text). Total dissolvable [Ba] and Ba-isotopic compositions are uniform for both stations at 69.7 ± 1.4 nM and +0.23 ± 0.02‰, respectively (±2 SD; n = 24); pBa constitutes 0.1–0.3% of total Ba

Fig. 3

Characterization of the pBa excess in Lake Superior. a–d from St. FWM; e–h from St. WM. Depth profiles of a, e p[Ba]_XS; b, f first derivative of p[P] with respect to depth, illustrating that the depths and magnitude of most intensive organic matter attenuation are correlated with the depths and magnitude of pBa_XS; c, g particulate Ba:Sr ratios, also shown are the dissolved (dashed line) and average crustal ratio (vertical bar); d, h Ba-isotopic data expressed as the difference from dissolved Ba-isotopic compositions; Δ^138/134Ba_{part.−diss.} = δ^138/134Ba_part. − δ^138/134Ba_diss.. Isotopic fractionation factors between particulate and dissolved Ba are shown as i, ii, iii, and iv for: inorganic barite precipitation, mean Lake Superior p[Ba]_XS (this study), regression of seawater data^–, and marine particles from the upper water column of the South Atlantic (Supplementary Fig. 7), respectively. Vertical error bars on the shallowest samples in d, h denote depth ranges over which particulate samples were pooled to obtain sufficient Ba for isotopic analysis. Horizontal error bars for any given property measurement reflect the propagated 2 × SD uncertainty. Shaded region highlights depth interval over which organic matter attenuation exceeds −1 nM P m⁻¹

Fig. 4

Conceptual model of barium cycling in low-sulfate water columns. Depth profiles of p[P] and p[Ba]_XS from St. FWM. (1) Autotrophic production leads to a peak in OM (organic matter) near the surface, indicated by the maximum in p[P]. (2) Microbial respiration within aggregates of decaying OM leads to development of barite-supersaturated microenvironments and precipitation of barite. (3) Continued respiration diminishes OM concentrations and destroys protected microenvironments, preventing further build up of Ba²⁺ and sulfate ions and thus any additional barite formation; settling barites are exposed to undersaturated waters and may start to dissolve

Fig. 5

Barite saturation as a function of organic matter remineralization in Lake Superior. Curves illustrate the evolution of Ω_barite in 1-L solutions assuming that both Ba and sulfate, only sulfate, or only Ba are able to accumulate following remineralization. To achieve Ω_barite = 1 in the Ba and sulfate scenario, between 92 and 95% of sulfate and 67 and 75% of Ba ions in the environment of barite precipitation must be derived from remineralization. Though the absolute quantities of organic matter requiring remineralization are dependent on the geometry and aggregate volume of microenvironments, the proportionality determined by these calculations implicates respired organic matter as the dominant source of Ba and sulfate in driving pelagic barite saturation in Lake Superior

Fig. 6

Isotopic offsets for barites precipitated under low ambient sulfate imply Archean seawater possessed heavy Ba-isotopic compositions. Using Δ^138/134Ba_XS−diss. = −0.41 ± 0.09‰ (this study), Archean barites from the Dresser Formation imply a dissolved Ba source with δ^138/134Ba_NIST = +0.37 ± 0.09‰, similar to the range reported for modern seawater^–, and considerably offset with respect to igneous rock standards (data are for AGV-1, G-2, BHVO-1, QLO-1, BIR-1, JG-1a, JB-1a, JR-1, and JA-1). Values for NBS-127, a barite standard reference material commonly used for S- and O-isotopic normalization, were determined as δ^138/134Ba_NIST = −0.27 ± 0.02‰ (±2 SD, n = 4), which we report here as a reference value for future studies. Shading indicates mean ± 2 SE for each sample set